Japanese Patent No. 3465862, Japanese Patent No. 4111651, and Japanese Unexamined Patent Application Publication (JP-A) No. H07-25296 describe manufacturing of automobile reinforcement members such as door beams and bumper reinforcements by subjecting an aluminum alloy extrudate to crushing, where the extrudate includes a pair of flanges arranged to face each other and two or more webs connected to the flanges, and the crushing is performed on an edge region of the extrudate in a direction perpendicular to the flange face. Such crushing has been believed to be desirably performed after aging from the viewpoints of working accuracy and cost. Typically, Japanese Patent No. 4111651 describes that crushing is performed after aging on a 6xxx series aluminum alloy extrudate formed through press quenching.
In contrast, 7xxx series aluminum alloy extrudates have inferior formability after aging, and, when subjected to crushing after aging, suffer from cracking in a web undergoing bending deformation, even when the crushing is performed at a low crushing rate (percentage reduction in cross-section height). This is because such 7xxx series aluminum alloy extrudates contain large amounts of alloy elements such as Zn, Mg, and Cu and thereby have higher strengths after aging than those of other alloy series. This tendency is more remarkable in higher alloys. To prevent this, JP-A No. 2003-118367, for example, describes that an extrudate after extrusion in a state of T1 temper is desirably subjected to crushing and subsequently to aging.
The 7xxx series aluminum alloy extrudates, however, undergo hardening due to natural aging and have inferior formability even when they are in state of T1 temper after press quenching and before aging. To improve the formability, reversion treatments have been performed to reduce the strengths of 7xxx series aluminum alloys which have been hardened due to natural aging, as described typically in JP-A No. H07-305151; JP-A No. H10-168553; and JP-A No. 2007-119853.